This invention relates to method and apparatus for measuring any of a large number of characteristics of lamplight and, more particularly, to a method and apparatus for expressing as a number any one of a large number of characteristics of artificial illumination as that characteristic is perceived by the average observer.
Illumination quality and quantity are best judged by consensus, using a group of expert, unbiased, experienced human observers with normal color vision. Given such a group of observers, one can expect to get a reliable evaluation of any aspect of illumination. After all, "illumination" has little meaning except as it relates to human activities, and as judged by human observers.
Is the office lit brightly enough for painstaking paper-work? Does the restaurant illumination make food look appetizing? Does the concert hall enable a patron to read his program and yet highlight the orchestra on the stage? Does the lighting in the hospital room encourage the patient by benefitting his appearance, to his own satisfaction and that of his visitors? Does the hotel lobby lighting yield attractive coloration of guests and decor? Will the draftsman in the proposed engineering complex be able to work long hours without visual fatigue?
A group of expert observers, with time and patience, can answer these questions. The human visual system can make such judgements, and of course helped formulate the questions in the first place. But such an investment in man-hours is seldom possible.
Is there a substitute for the group of human observers, a sort of "secondary standard"? Consider the complete spectral power distribution of the illumination, i.e. the composition of the lamplight, in one of the situations above. A first-class spectroradiometer can determine the spectral content of the lamplight. Once that is accomplished, a first class computer can evaluate the content of the lamplight, and come up with ratings which partially answer many questions like those above.
It is now possible that a portable instrument can do better even than that. What is needed: (1) An optical device simpler than the spectroradiometer, (2) more sophistication in relating the results to what is actually seen, and (3) the same first class computer.
An illumination quality meter would look at the lamplight, but see much of what the human sees when he looks at the illuminated scene. Among other things, the meter should see brightness, footcandles, attractiveness of coloration, color, color-temperature, color rendering index, gamut of coloration, fading risks, color-scheme stability, visibility. It should indicate how many brightness units are equivalent to every footcandle, and to every watt of light; how many visible watts fall on a square meter of work surface. It should indicate how stable color-schemes will be in a proposed illumination. The lamp-user should know exactly what his customers will enjoy about the lighting--and what they will find distasteful.
Among the important qualities of illumination, particularly artificial illumination as supplied by commercial lamplight, are: 1. Brightness, as perceived by the user, and 2. Coloration of the scene, (a) as judged by the trueness of the observed colors, (b) as judged by the pleasantness and satisfaction given by the colors of the surroundings, (c) as judged by how clearly colors are seen. None of these three qualities of illumination has been adequately measured by meters available to the user. The familiar footcandle meter can differ by 100% or more from expert visual judgement of the brightness of a scene, while no good measure at all of visibility was possible. The Color-Rendering Index (CRI), which can be calculated with the use of a full scale computer, is an index of similarity of coloration to that afforded by real phases of daylight; it fails to agree with the observer's assessment of coloration of a scene in all three respects, trueness, pleasantness, and clarity.
Some lighting problems with which a new meter should cope: Assess particularly pleasing and successful lighting installations, to determine which qualities are the important ones in those cases. Assess a problem installation, to determine what qualities are lacking. Assess potential customers' present lighting, to establish a basis on which to make recommendations for improvement. Assess a new lamp, independent of its environment, to predict, before installation, the quality of its illumination. Assess different qualities and quantities of illumination, under which difficult visual tasks (a) are done easily and comfortably, (b) are done with difficulty, or (c) should not be attempted. Assess many lighting environments, to build up a personal correlation between what the user sees and the numerical qualities measured by the meter. Choose lamplight for minimum ultraviolet and violet content per unit of brightness or visibility. Act as a portable monitor, to assure that certain illumination fulfills requirements either as to quality or quality, or both. Act as a portable monitor, with analysis capability unequalled even by the best (portable or stationary) spectroradiometer. Act as a portable monitor, substituting for a group of trained observers at the user's elbow. Act as a portable monitor, substituting for the customer's own visual system. Act as a portable monitor, demonstrating that all requirements in lighting design have been fulfilled. Act as a portable monitor, showing what lighting requirements are still lacking before the installation is turned over to the customer. Indicate which lamplight yields the most visibility per watt of lighting power; how to save the most electrical-power cost; the most energy, in kilowatt-hours or barrels of oil. Indicate how rapidly lamplight may be expected to fade furnishings.
Some places to use the new meter: The kitchen: clarity and appearance of food during preparation need monitoring. The museum, or art gallery: are the precious artifacts adequately visible, but minimally irradiated by harmful wavelengths? The factory: difficult visual tasks can be done with comfort and well-being, if the lamplight is monitored and improved in quality. The postoffice: sorting is the sort of visual task requiring exceptional "seeing". The bank: it has already been demonstrated that banking tasks can be done in visual comfort, with half the electric power, if lamplight quality is sufficiently high. The mill: Aristotle found some little time ago that weavers need special care in their illumination. The buffet: in the home, it is sufficient if the guests find the repast appetizingly colored and attractive; in the commercial restaurant or hotel, it is a matter of economics that the paying diners find the color-rendering of the food array enticing. The restaurant kitchen: crisp clarity of vision helps get the right ingredients in the right bowls. The diagnostic area: in the home, doctor's office, or hospital it is essential that the lamplight render the patient's condition accurately. The beauty parlor: make-up application, hair-coloring, finger nail polish application all require good color-rendering and clarity. The barber shop: rendering of hair and complexions, as well as good-seeing for the barber, are economic requirements. The hotel or motel bathroom: color-rendering of the usual standard fluorescent lamp takes out all the enthusiasm of a new day. Make-up stations, rest rooms, vanity mirrors in hotel and motel rooms: lamplight effects on complexion colors may be the most important of all. The restaurant: color-rendering of both food and patron, and at the same time achieving a subdued lighting-level, is a real art. The laundry (home or professional): how white is white? Without proper color-rendering, it is impossible to tell. The food-processing plant, or cannery: does every bit of material belong in the can? The fabric store: what is the color scheme in the plaid really like? How does a fabric-color really relate to its neighbors?
The usual light meter outputs a single number. Sometimes, but not always, that single number gives the user an accurate measure of how bright a space appears to the average person, or how well one can see in the space, or what camera exposure to use. The meaning of its scale of units must always be carefully and patiently learned. If the meter is a "footcandle meter", experience teaches the user this sort of thing: (1) a reading of 1 footcandle warns that some difficulty probably will be encountered in reading a newspaper; (2) a reading of 50 footcandles is likely to be comfortable; (3) a reading of 500 footcandles will sometimes seem too bright. Not much more than that can be inferred from reading a footcandle meter. The footcandle meter is an inadequate stand-in for human vision, or for the human observer. Even so, its scale of units takes time to understand and use.
The footcandle-meter has a single eye, as does the light-meter on a camera. With a single eye, a light-meter can distinguish lightness and darkness--nothing more. What a single eye senses can be described with a single dimension, like inches on a yardstick.
U.S. Pat. No. 4,334,782 dated June 15, 1982, to Thornton discloses method and apparatus for expressing as a number the relative brightness of artificial illumination as it is perceived by the average observer. There is extracted (i.e., detected) from the illumination to be measured, a blue-appearing narrow band, a green-appearing narrow band, a yellow appearing narrow band; and a red-orange-appearing narrow band. From these extracted bands are generated six different signals related to the watts of energy in each of the four extracted bands. The six signals are combined and expressed as a number which is indicative of the brightness of the artificial illum-ination as perceived by the average observer. This four eyed device agrees better with what the normal human observer sees as "brightness"than does the footcandle-meter described above.